Common bleaching agents generally fall into two broad classes: (1) iron-based bleaching agents, and (2) peracid bleaching agents. Examples of the first class include ferricyanide, ferric complexes of ethylenediaminetetraacetic acid (EDTA) and 1,3-propylenediaminetetraacetic acid (PDTA), and the ferric complex of beta-alaninediacetic acid (ADA). Ferricyanide has excellent silver bleaching capability, but once released into the environment, it can cause aquatic toxicity due to the photochemical liberation of free cyanide ion. Ferric PDTA is likewise a good silver bleaching agent, but PDTA is not readily biodegradable. Ferric ADA uses a biodegradable ligand, but its strength as a bleaching agent is inferior to that of FePDTA. Therefore, it must be used in higher concentrations which are undesirable for cost and environmental reasons. In general, iron-based bleaching agents also have the environmental disadvantage of contributing relatively high concentrations of iron to photographic effluent. A growing number of regulations, particularly in Europe, restrict the discharge of iron from photofinishing operations.
Examples of the second class of bleaching agents include three distinct subclasses: (a) hydrogen peroxide and peroxide precursors such as perborate and percarbonate, (b) persulfate acids and salts, and (c) perhalogen acids and salts, such as chlorate, bromate, iodate and perchlorate.
The perhalogen bleaching agents are not often used for silver halide color photographic systems because of their tendency to degrade dye images. Hydrogen peroxide bleaches generally have excellent environmental properties, but all peroxide bleaching compositions described to date suffer from one or more deficiencies (see for example, U.S. Pat. No. 4,277,556 of Koboshi et al, U.S. Pat. No. 4,301,236 of Idota et al and U.S. Pat. No. 4,717,649 of Hall et al). Such deficiencies include incomplete silver bleaching, incomplete retention of bleached silver in the element, vesiculation (formation of small bubbles and pinholes from release of oxygen) in the element, and inadequate stability of bleaching solutions during storage and use.
Persulfate bleaching agents, especially ammonium persulfate and sodium persulfate, are used in some commercial photographic process (such as the Eastman Color Print process for motion picture film), but these processes require a separate bleach accelerator bath, the active ingredient of which, is an alkyl thiol, which has a foul odor.
Metal-catalyzed persulfate bleaching solutions avoid the need for a thiol bath and generally require much lower metal concentrations than bleach solutions containing iron-based bleaching agents. However, such bleaching solutions have significant limitations. Research Disclosure publication 15704 (Vol 1.157, May 1977, page 8) teaches the use of a variety of metal complexes as catalysts for persulfate bleaching. With the exception of iron and perhaps manganese, the aquatic toxicity of the metal ions themselves precludes the practice use of such complexes as bleaching agents. The one ferric complex disclosed in this publication, iron complexed with 2,2'-bipyridine, requires a prohibitively expensive ligand and has a tendency to be retained in the photographic element, leaving an undesirable pinkish-red stain.
The bleaching agents described in DE 3,919,551 A1 slowly and incompletely bleach photographic elements with substantial contents of silver bromide and silver iodide. Another disadvantage of these bleaching solutions is that they exhibit the best bleaching performance at below pH 3 where persulfate undergoes acid-catalyzed decomposition. This results in poor stability of the bleaching solutions.
Useful iron-catalyzed bleaching solutions are described in copending and commonly assigned U.S. Ser. No. 08/230,189 (filed Apr. 20, 1994, by Buchanan et al). These bleaching compositions offer excellent silver bleaching and good stability, but further improvements are needed because the preferred ferric catalysts have low water solubility and sometime result in the formation of crystalline solids in bleaching and replenisher solutions.
Japanese Kokai 51-07930 (published Jan. 22, 1976) describes the use of nitrolotriacetic acid or 2,6-pyridinecarboxylic acid or both to reduce stain in ferric-based bleaching solutions. The publication teaches that stain reduction is achieved equally well when the ligands are included in the bleaching solution, in the bleaching solution and neutralizing bath, or in the fixing bath. This reference therefore teaches away from the criticality of these ligands or their iron complexes as silver bleaching agents. Moreover, there is no mention of peracid bleaching agents.
Japanese Kokai 53-048527 (published May 2, 1978) describes the use of bleaching solutions containing aminopolycarboxylic acid metal complexes and/or a polycarboxylic acid metal complex salt (such as a 2,6-pyridinedicarboxylic acid salt). The preferred metal complex is FeEDTA, and the alleged advantage is reduced fog and high color image density. There is no suggestion of silver bleaching advantages or the use of peracid bleaching agents.
Japanese Kokai 50-26542 (published Mar. 19, 1975) describes bleaching solutions containing an iron chelate with one or more ligands such as 2-carboxypyridine, 8-hydroxyquinoline or 2-carboxypyrazine. These solutions fail to provide the rapid and superior bleaching performance desired in the industry. Furthermore, this publication teaches the use of very high iron concentrations (for example, 0.554 mol/l in Example 1), and makes no mention of peracid bleaching agents. It therefore teaches away from the use of low concentrations of iron complexes to catalyze peracid bleaching agents.
Bleaching solutions have been developed which contain more than one ligand and which help provide rapid bleaching without unwanted dye formation in color photographic materials. However, such solutions contain two distinct iron-complex salts. For example, in KODAK FLEXICOLOR.TM. Bleach II, one salt is ferric ammonium-EDTA, and the other is ferric ammonium-PDTA. While such mixtures are stable and provide excellent bleaching, neither of the noted complexes is readily biodegradable. Other mixtures of complexes are described in EP-A-0 430 000, but they lack stability when used in combination with thiosulfate fixing agents. Other ligand mixtures are described in EP-A-0 534 086 wherein bidentate ligands are used as buffering agents.
Useful ternary bleaching agents are described in copending and commonly assigned U.S. Ser. No. 08/128,626 (filed Sep. 28, 1993, by Gordon et al). Such materials comprise one iron atom and two different ligands. While these materials are useful in some processes, there continues to be a need for more rapid processes using biodegradable materials. Moreover, they are restricted to use in bleach-fixing solutions.
There remains a need in the art for highly water-soluble catalysts for peracid bleaching solutions which catalysts preferably comprise biodegradable ligands, provide rapid bleaching and are compatible with chloride rehalogenation.